We have developed heat resistant strain sensors using laser processing techniques. The application is aimed at structural health monitoring for high temperature piping systems. This situation requires extraordinary durability such as radiation resistance and noise isolation due to adverse conditions caused by nuclear reactions or electro-magnetic pulses. We proposed that a Fiber Bragg Grating (FBG) sensor made by femtosecond laser processing could be the best candidate. The combination of fabric reinforcement and a heatproof adhesive mold successfully protected the fragile optical fiber once the fiber was installed on the piping material’s surface. To make the best use of the heat-resistant characteristic, we fixed the FBG sensor by metal mold. A groove was processed onto the surface of a SUS metal plate with a grindstone. We used a Quasi-CW laser to weld a filler wire onto the plate. The optical fiber was situated under the filler wire before was heated by laser pulses with 10 joule energy and a duration of 10 ms. A series of weld pool formed a sealing clad on the groove. The FBG sensor was buried at a depth of 1 mm over a length of 1 cm. No degradation in its reflection spectra was detected before and after the processing. The FBG sensor can detect the vibration of the plate caused by impact shocks. In this paper, the Bragg peak shift of the FBG sensor under laser cladding condition has been discussed.
We have been developing a new laser cladding system to repair the damages of parts in aging plants. It consists of some devices which are a laser torch, composite-type optical fiber, QCW fiber laser and etc. All devices are installed in a mobile rack, so we can carry it to plants, laboratories or anywhere we want to use. We should irradiate the work with the best accuracy of laser beam and filler wire in laser cladding. A composite-type optical fiberscope is useful. This fiberscope was composed of a center fiber for beam delivery surrounded by 20000 fibers for visible image delivery. Thus it always keeps target on center of gun-sight. We succeeded to make a line laser cladding on an inside wall of 1-inch tube by our system. Before this success, we solved two serious problems which are the contamination of optics and the deformation of droplet. Observing laser cladding process by X-ray imaging with Spring-8 synchrotron radiation, we found that the molten pool depth was formed to be under a hundred micrometers for 10 milliseconds. A Quasi-CW fiber laser with 1 kW was employed for a heat source to generate the shallow molten pool. The X-ray shadowgraph clarified that a molten droplet was formed at the edge of a wire up to a millimeter size. It grew up if the wire didn’t contact with the tube wall in initial state. Here we succeeded to measure the thermo-electromotive force voltage between a wire and a tube metal to confirm whether both came in contact. We propose to apply the laser cladding technology to the maintenance of aging industrial plants and nuclear facilities.